Method and apparatus for separating and recovering particulate material

ABSTRACT

Particulate material such as coal of a size inferior to 1 mm is recovered by being entrained in a liquid medium and being fed through an apparatus comprising a plurality of cyclone separators connected in series, the cyclone angle of the separators decreasing in the direction of flow. The cyclone angle α 2  of a cyclone separator in the series is within ±5% of the angle calculated according to the formula ##EQU1## where α 2  is the cyclone angle of a cyclone separator following a separator with angle α 1  and φ 1  and φ 2  represent the geometric mean particle size of the cut of successive separators with these angles.

This is a division of application Ser. No. 269,920, filed 11-10-88 nowPat. No. 4,865,740.

TECHNICAL FIELD

This invention relates to improvements in the method of and apparatusfor separating and recovering particulate material.

BACKGROUND ART

There are many applications where the separation and recovery ofparticulate material from bulk waste material and untreated run of themine material (R.O.M.) would be economically and ecologically desirable.An example of this is the mounds, tips or slag heaps erected and beingerected by the mining industry and composed of what was or is consideredto be waste material produced in the winning of a mineral from the mine.Such tips, slag heaps or ponds contain quite large proportions of themineral concerned but in a form which, at the time the reject reservewas being created, it was not considered viable to recover. Generallyspeaking, the mineral in the slag heaps and ponds is of particulate formand of such a small size that it was not thought possible to recover iteconomically.

DISCLOSURE OF THE INVENTION

The present invention provides a method of and apparatus for recoveringand separating particulate material of the small sizes in question sothat not only can the minerals concerned be recovered but the slag heapsand ponds can be demolished and re-distributed in the process to removean unsightly presence from the landscape.

Of particular, but not exclusive, concern is the recovery of coal fromslag heaps and ponds. The amount of coal present in such a reservevaries from area to area, but we have found that 20 to 100% of the coalpresent is of a particle size which is 1 mm or less. Coal of this sizehas not hitherto been believed to be recoverable commercially. We havenow found that the major part of this coal, e.g., 70 to 100% by weightcan be recovered using cyclone separators.

It is known to use cyclone separators for separating and recoveringgranular or particulate material. Such separators comprise an uppercylindrical portion having a coaxial tube (vortex finder) therein and afrusto-conical portion extending away from the cylindrical portion tothe smaller diameter outlet or underflow. Material to be separated ismixed in a liquid medium to form a slurry and is introducedsubstantially tangentially into the cylindrical portion so that itspirals about the tube moving towards the underflow. After passing theinner end of the tube a proportion of the medium changes direction andflows upwardly through the tube to exit at the overflow, taking with itthe particles of lesser density and size, the remainder exiting throughthe underflow.

The maximum density of the mineral to be recovered is generally known.In the case of coal the density is in the range of 1.5 to 2.5 and, moreusually, about 1.8 to 2.0. It has hitherto been considered not possibleor practical to recover by mechanical means only coal in this densityrange and having a particle size of 1 mm or less. Our research has shownthat conventional cyclone separators operated in a conventional way donot separate coal particles of 1 mm or less in the density range of 1.8to 2.0 from the waste material in which they are to be found. Ourinvestigations have further revealed that the "cut" or characteristic ofa cyclone separator can be modified by changing or selecting the cycloneangle. For present purposes, the cyclone angle is considered to be theangle a generatrix of the frusto conical part subtends to the axis in acommon plane. By reducing this angle one can reduce the size of theparticles which exit at the overflow having a given density or densityrange. If one assumes the cyclone angle to be α and the geometric meanof the particle sizes exiting at the overflow to be φ, then theexpression

    K.sub.1 +K.sub.2 ·cot (90-2α)=φ         1

appears to hold true in the density range for coal. As K₁ is very small(close to 0), 1 can be written

    cot (90-2α)=Kφ                                   2

Thus, if one were looking to cut or separate coal particles in the rangeof 250 to 500μ, then

    φ=√250×500μ≈350μ.

According to one aspect of the present invention there is provided amethod of separating and recovering particulate material which comprisesfeeding the material entrained in a liquid medium through a plurality ofcyclone separator stages connected in series by their overflows, thecyclone angle of the separators of the respective stages decreasing inthe direction of flow. As each stage can be one or more similarseparators connected in parallel, each stage will be simply referred toherein as a cyclone separator.

It will be appreciated that the bulk or size of a slag heap or tip orR.O.M. is such that the apparatus of the invention would generally beassembled on site and that samples would initially be taken of the rawmaterial in the tip to ascertain density and particle size distribution.From this, the cyclone angle α₁ of the first cyclone in the series canbe arrived at given the estimated geometric mean particle size φ₁ forthe first cut. One can then decide on the range of particle sizes forthe cuts of successive separators. From equation 2 above it will be seenthat ##EQU2##

Having decided initially on the values for α₁, and φ₁, and havingselected φ₂ for the second separator, it will be seen that the optimumcyclone angle α₂ for the second separator can be calculated.

In the preferred method of the invention, the cyclone angle α₂ of acyclone separator in a series is ±5% of the angle calculated by theformula ##EQU3## where α₂ is the cyclone angle of a cyclone separatorfollowing a separator with angle α₁ and φ₁ and φ₂ represent thegeometric mean particle size of the cut of successive separators withthese angles.

The liquid medium in which the particles are entrained is preferablywater and it should be noted that the angle α controls the size range ofwashed or clean particulate material reporting to the overflow, φ beingthe geometric mean of this range.

It will be appreciated that for a given input velocity of the material,the larger and more dense particles report to the underflow and thesmaller and lighter particles report to the overflow. The particulatematerial reporting to the overflow will therefore comprise a definedrange of particle sizes of the mineral which are washed or clean and abalance of particles of smaller sizes which are not washed. By selectingthe angle of the next succeeding cyclone separator, the washed particleswithin the range from the preceding cyclone separator report to and arerecovered from the underflow and a further range of smaller particleswhich have been washed report to the overflow for recovery at the nextsucceeding cyclone separator.

If one assumes M to be the maximum size in microns of particles to berecovered, m to be the minimum size in microns, R to be the ratio of theupper and lower limits of particle sizes in the range to be cut in eachseparator, then the number of stages required would be ##EQU4## Forexample, if the particulate material to be recovered is in the range ofM=1000μ and m=30μ and R=2 then ##EQU5##

Each of the 5 stages would provide a range of washed particles reportingto the overflow as follows,

    ______________________________________                                        Stage 1:      500-1000μ                                                    Stage 2:     250-500μ                                                      Stage 3:     125-250μ                                                      Stage 4:      62-125μ                                                      Stage 5:     31-62μ                                                        ______________________________________                                    

If M=500μ, m=30μ and R=2 then N=4 and this represents the number ofstages in the example which follows.

If one assumes an arrangement with N stages and uses the suffix i toidentify a stage intermediate stage 1 and stage N then ##EQU6## φ_(i)being the geometrical mean of the range of particle sizes cut by stagei.

The method of the invention may be regarded as separating and recoveringparticulate material by feeding the material entrained in a liquidmedium to the first of a series of cyclone separators having a cycloneangle selected to cause clean and washed particles within a firstdefined range of particle sizes to report to the overflow together withparticles of lesser size and feeding the material recovered from theoverflow to a second cyclone separator having a cyclone angle less thanthat of the first separator and selected to encourage the cleaned andwashed particles within the first defined range to report to theunderflow for recovery and to cause cleaned and washed particles withina second defined range of particle sizes less than those of the firstdefined range to report to the overflow.

It will be appreciated that according to another aspect of the inventionthere is provided an apparatus for separating and recovering particulatematerial which comprises a plurality of cyclone separators connected inseries with means for supplying the particulate material entrained in aliquid to the inlet of the first separator of the series and means forsupplying the overflow from each separator to the inlet of the nextseparator, the cyclone angle of each cyclone in the series being lessthan that of its predecessor in the series.

With advantage, the cyclone angles of successive cyclone separatorsconform to the formula ##EQU7## where α₁, α₂, φ₁ and φ₂ represent theparameters identified above in relation to equation 3.

It will be appreciated that for a given specific gravity (S.G.) thecoarser particles report to the underflow of a separator and the finerparticles to the overflow, although there will inevitably be some thatreport to the wrong exit. Our research has further identified that thediameter of the cyclone is a parameter which affects the efficiency ofoperation of a cyclone separator in relation to the particle sizes whichreport to the underflow or which exit through the overflow. By selectingthis diameter it is possible to achieve a higher efficiency for thefiner or smaller particles reporting to the overflow than for the largerparticles reporter to the overflow. Thus, by selecting the cycloneangle, one can select a range of particle sizes to report to theoverflow and, by selecting the cyclone diameter, one can reduce thechance of larger particle sizes reporting to the overflow and enhancethe quantity reporting to the underflow.

With a plurality of cyclone separators in series, decreasing thediameter of the cyclones from one to the next has the effect ofenhancing the quantity of coarser particles reporting to the underflowwith respect to the quantity of coarser particles undesirably reportingto the overflow.

In its preferred form, the invention provides a method of separating andremoving particles from waste material and unprocessed R.O.M. whichcomprises feeding the raw material suspended in water through aplurality of cyclone separators in series connected by their overflow inwhich both the cyclone angle and the diameter of the cyclones decreasefrom one separator to the next.

The invention also provides an apparatus for separating and removingparticles from waste material and R.O.M. which comprises a plurality ofcyclone separators arranged serially and having both a cyclone angle anda diameter smaller than the preceding cyclone separator in the seriesand means for feeding the waste material entrained in water to the firstseparator in the series.

It will be appreciated from the above discussion that the presentinvention provides a plurality of serially connected cyclone separatorseach of which has a cyclone angle selected to encourage coarserparticles in a range of sizes to report to the underflow, which range iswithin the particle size of the finer particles reporting to theoverflow of the preceding separator and each of which separators has adiameter which decreases from cyclone separator to cyclone separator inthe direction of flow in the series and which enhances the quantity ofcoarser particles in the range of sizes reporting to the underflow ofthe separators.

The efficiency E_(i) of a stage is considered to be the ratio of theactual weight of particles of size φ_(i) reporting to the overflow ofstage i cyclone with respect to the theoretical weight which shouldreport in ideal conditions at the desired maximum specific gravityadmitted in the clean product.

For cyclone stages 1 to N-1, the efficiency should be at least 0.9whereas for last stage N it should be not greater than 0.05.

E_(i) is a function of the diameter of the cylindrical part of thecyclone separator so that this diameter D_(i) is an important parameterin optimizing the method and apparatus of the present invention.

In fact for cyclone stage 1 to N-1, the diameter D_(i) is given by theequation ##EQU8## where D_(i) is in inches and φ_(i) is in microns. Forthe last stage D_(N) where i=N we have ##EQU9##

The underflow at stage i cyclones is composed of wanted, clean, washedproducts of the desired S.G. and of a size equal to or greater thanS_(i) together with unwanted ashy products of a size equal to or lessthan S_(i).

S_(i) may be considered to be the cut size to be made by a screen placedbelow the underflow of stage i cyclone.

    S.sub.i =φ.sub.i ·√R                   7

It will be noted that with N=5; R=2. M=1000μ and m=30μ. The followingvalues for S_(i) are given

    ______________________________________                                        underflow of stage                                                                           cut size of screen (μ)                                      ______________________________________                                        1              990μ                                                        2              495μ                                                        3              247μ                                                        4              123.7μ                                                      5              61.9μ                                                       ______________________________________                                    

It is desirable that the input velocity of the material slurry should besubstantially the same at each stage and should be between 1.4 to 1.75m/sec.

It is also to be preferred that the diameter of the underflow outletshould progressively decrease from one separator to the next in theseries. The diameter of the underflow of a separator stage can be variedto adjust the performance of a stage to provide a degree of fine tuning.

The invention provides an apparatus designed to "cut" or separate at apredetermined S.G., between 1.5 and 2.5, an assembly or collection ofparticles inferior to 1 mm in size, using a plurality of cyclonesconnected in series by the overflow and feeding adequately chosenscreening arrangements by the underflow.

Such apparatus is operable to wash particles of a size between M and mmicrons where M is not greater than 1 mm by cutting or separating atd_(M) (maximum S.G. admitted in the clean product). For a given d_(m)the ratio ##EQU10## washed by any cyclone of the series varies with thewashability of the raw material. For a given application R has adetermined value comprised between 1 and 5.

One embodiment of the invention will now be described by way of example,reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a typical arrangement for recovering largersized coal;

FIGS. 2A and 2B together represent a flow diagram of an arrangementaccording to the present invention for recovering coal particles of 1 mmand less in size;

FIG. 3 is a continuation of FIGS. 2A and 2B; and

FIG. 4 is a graph which is illustrative of the principle of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The example to be described is concerned with the recovery of coalparticles of a size of 1 mm or less from raw material which is usuallyavailable in bulk as slag heaps, tips or ponds adjacent worked-out coalmines. Commonly such reserves will contain between 20 and 80% by weightof coal of a particle size of 1 mm or less which was not previouslybelieved to be economically viable to retrieve.

In order to avoid the expense of transporting the raw material, it isintended that the coal particles should be separated at the site of thereserve. The apparatus of the present invention can therefore be custombuilt on site to suit the characteristics of any particular rawmaterial. The raw material to be treated is therefore initially examinedto obtain a measure of the quantity of coal particles in given sizeranges and their distribution within the raw material. The apparatus isthen designed and assembled.

As illustrated in FIG. 1, the raw material is delivered to a feed trough1 from which it passes to a distributor 2 which delivers it to a furtherconveyor 3 which conveys it to a vibrating screen 4. Water is sprayed onto the material on the screen 4 and is supplied along pipes 10 from atank 10a. The large sized material of the order of 25 to 120 mm passesdown a spout 5 to a conveyor 9 which conveys this material to a dump. Apermanent magnet 6 is associated with the spout 5 to remove any ironthat may be present. Smaller sized pieces are diverted through a crusherand sizer 7 to a conveyor 11 for recycling to the conveyor 3.

The material passing through the vibrating screen 4 is passed to a sievebend 8 which separates the particles into larger and smaller sizes suchthat the smaller sizes of about 1 mm and less are passed to a deslimingsump 101 (FIGS. 1 and 2B). The larger sizes are passed over a deslimingscreen 12 which allows smaller sizes which are trapped to escape to thedesliming sump 101. The larger sizes travel on from the screen 12 to adense medium cyclone washer which separates coal from the clay and otherundesirable impurities present. Such coal would generally be greaterthan 1 mm in size and usually about 1 to 25 mm.

The desliming sump 101 represents the source from which coal particlesof a size of 1 mm and less are to be recovered. The slurry in sump 101is pumped by pump 102 (FIG. 2) through manifold 103 to the inlet ofcyclone separator 104 which is the first of a series of four stages ofcyclone separators in this example represented by separators 104, 112,118 and a multicyclone ring of separators 125. The material entrained inwater is fed to each of the separators 104, 112, 118 and 125 at the samevelocity.

The cyclone separator 104 is intended to wash through the overflowwashed, clean coal particles of a size of 250μ to 500μ of the desiredS.G. together with unwashed particles of lesser size. It has a cycloneangle of 31° and a cyclone diameter of 30 ins.

It is intended to remove by the underflow particles of a size of 0.5 to1 mm which are unwashed and are passed to the dense medium cyclonewasher previously referred to and which receives the larger sizes fromthe screen 12.

The particles size of 0.5 to 1 mm could have been washed in anadditional cyclone separator preceding the separator 104 and having adiameter of 35 ins and a cyclone angle of 38°. This would bring thenumber of cyclone separators to five in conformity with equation 4.

The cyclone separator 112 is intended to wash through the overflowwashed, clean coal particles of a size of 125μ to 250μ. It has a cycloneangle of 21° and a diameter of 25 ins. The washed, clean particles inthe range of 250μ to 500μ from the previous stage 104 report to theunderflow for recovery.

The cyclone separator 118 is intended to wash through the overflowwashed, clean coal particles of a size of 65μ to 125μ. It has a cycloneangle of 10° and a diameter of 20 ins.

The washed, clean particles in the range of 125μ to 250μ from theprevious stage 112 exit through the underflow for recovery.

The cyclone separators 125 are intended to remove by the underflow coalparticles of a size of 65μ and coarser. They have a cyclone angle of 3°and a diameter of 9 ins.

The first cyclone separator 104 is effective to remove from the slurrythe finer coal particles roughly of the side of 0 to 0.5 mm and causethese to exit through the overflow to the sump 109 from which this finerslurry is pumped by pump 110 to manifold inlet 111 to the second cycloneseparator 112.

FIG. 4 shows some characteristic curves A for different cyclone anglesof plots of density to particle size reporting to the overflow. It willbe seen that the cyclone angle of 31° offers a reasonable compromise inorder to exit at the overflow the majority of particles of a size lessthan 500μ with particles of the specific gravity concerned in the sizerange of 250 to 500μ (geometric means 350μ) in the washed condition. Onthe same graph can be seen curves B representing efficiency relative toparticle size reporting at the overflow. It can be seen that thisefficiency falls away for particle sizes of 500μ and upwards and thatthe curve B for the cyclone angle of 31° again offers the bestcompromise for encouraging the particles of a size greater than 500μ notto report to the overflow but rather to the underflow.

As previously mentioned, the larger particles of size 500μ to 1 mm exitfrom the underflow 104 as unwashed coal and pass over a sieve bend 106followed by a vibrating screen 107 to a conveyor 108 and are conveyed inthis example to the dense medium washer previously referred to. Theunwanted clay and slurry is taken from the screen 107 to an additionalhydrocyclone preceding the separator 104 as previously mentioned so thatthe latter would be fed with washed, clean coal in this size range.

The succeeding cyclone separator stages preform similar operations, theparticle size being separated reducing with each separator. Thus cycloneseparator 112 sends washed coal particles of a size of 250-500μ from itsunderflow to the vibrating screen 114 and then through screw conveyor306 to centrifuges 308 which dry out the material. The unwanted clay andslurry is passed from the screen 114 to the tank 132.

The balance of the material exits from the overflow of the separator 112and comprises washed, clean coal particles in the range of 125-250μ andunwashed particles of lesser size. This material passes through sump 115and pump 116 to the manifold inlet 117 of the next cyclone separator118. Here, washed coal particles of a size in the range 125μ to 250μexit through the underflow through a tank 120 and a feeder 121 to acentrifugal screener 122 from which they are supplied to a conveyor 130.The unwanted clay and slurry is fed from the centrifugal screener 122 tothe tank 132.

The overflow from the separator 118 passes through sump 123 and pump 124to the multicyclones 125. Washed coal particles of a size in the rangeof 65μ to 125μ exit from the underflows through tank 127 and feeder 128to a centrifugal screener 129 whence they are supplied to the conveyor130. The conveyor 130 advances the particles of size 65μ to 250μ throughconveyors 131 and 305 to the screw conveyor 306 and hence to centrifuges308 which dry out the material. The dried material from the centrifugespasses along conveyor 313 to elevator 315 for collection. The unwantedwater or slurry is collected in tank 309 and pumped by pump 310 eitherdirectly to tank 132 or to a further cyclone separator 311 which gives afinal separation, the separated coal particles going through theunderflow to the screw conveyor 306 and the waste slurry going to thetank 132.

It will be appreciated that although ranges of particle size arereferred to, it is inevitable that there is an overlap in practice inthe particle sizes extracted at each separator. Nevertheless, byutilizing a plurality of cyclone separators in series and byconstructing these selectively to discard reducing bands of particlesizes, it is now possible economically to extract coal particles of 1 mmand less from waste tips and R.O.M. In consequence, the volume of wasteis materially reduced and unsightly slag heaps, tips and ponds can bereplaced by landscaping to improve the environment.

The waste slurry from the tank 132 is disposed of by dewatering inprecipitator 135 and pumping to an aqueous resting place such as a pondor lake (145, 151, 152) or even a river 143.

It will be appreciated that although a specific description is concernedwith the recovery of coal particles, the invention has application tothe recovery of other minerals which are to be found in large volumes ofpresently discarded material. Initially, one needs to ascertain thedensity and particle size of the mineral to be recovered and how a givencyclone separator responds to these parameters. It is then possible, byvarying the cyclone angle, the diameter of the cyclone and preferablyalso the diameter of the underflow outlet, to modify the cycloneseparator characteristics to suit the particular application in view.

The diameters of successive cyclones in the series are related by theformula ##EQU11## where K=1 for all but the last cyclone in the seriesand K=4.4335 for the last cyclone.

I claim:
 1. A method of separating and recovering particulate materialwhich comprises providing a plurality of cyclone separators connected inseries, the cyclone angle of the separators decreasing in the directionof flow and the cyclone angle α₂ of a cyclone separator in the seriesbeing ±5% of the angle calculated according to the formula ##EQU12##where α₂ is the cyclone angle of the cyclone separator following aseparator with cyclone angle α₁, and φ₁ and φ₂ represent the geometricmean particle size of the cut of successive separators with these anglesand feeding the material entrained in a liquid medium therethrough.
 2. Amethod according to claim 1 in which the diameter of the underflowoutlet in the separators decreases from at least one separator to thenext in the series.
 3. A method according to claim 1 or 2 in which thediameter of the successive separators decreases in the direction offlow.
 4. A method according to claim 3 in which the diameters ofsuccessive cyclone separators are related to the formula ##EQU13## K=1 lfor the (N-1) first cyclones and K=4.4335 for the Nth cyclone.
 5. Amethod according to claim 1 in which the particulate material entrainedin the liquid medium is of a size of 1 mm or less.
 6. A method accordingto claim 5 in which the specific gravity of the material is between 1.5and 2.5.
 7. A method according to claim 6 in which the specific gravityof the material is in the range of 1.8 to
 2. 8. A method according toclaim 5 in which the particulate material is coal and the liquid mediumis water.
 9. A method according to claim 1 in which the particulatematerial entrained in the liquid medium is injected into each of thecyclone separators of the series at substantially the same velocity. 10.A method according to claim 9 in which the input velocity is between 1.4and 1.75 m/sec.
 11. A method according to claim 1 in which the materialis fed through a series of N cyclone separator stages of which theefficiency of stages 1 to N-1 is at least 0.9 and the efficiency of thelast stage is not greater than 0.05.
 12. A method of recovering coalparticles of a size less than 1 mm from waste material and R.O.M. whichcomprises providing a series of cyclone separator each having a cycloneangle selected progressively to separate to the underflow a range ofprogressively smaller particles and collecting the ranges of particlesfrom the underflow of the separators and feeding the particles entrainedin water through said separators.